The present invention relates to a process for printing dots on a heat sensitive paper by using a thermal printer and thermal printer using the said process.
A process is known for printing dots on heat sensitive paper by means of a thermal printer, whose head has N resistors spaced by twice the spacing of the dot to be printed and aligned in the direction of the line to be printed.
According to the process, the printing of a line with 2N dots involves two stages. During the first stage, the printing head is kept stationary and the resistors corresponding to the dots to be printed are heated. During the second stage, the printing head is displaced by one spacing in the direction of the line in question, so that the resistors corresponding to the locations of the dots in the line still to be printed are heated. To prevent any streaks forming on the paper, the heating times for the resistors are spaced by the displacement times of the printing head and the displacement times of the paper beneath the printing head for positioning the latter on the new line to be printed. As a result of this process, the time for printing a line is constant and it is relatively easy to synchronize reception of the data with the device supplying the same. However, the data relative to a line can come from random devices, such as computer off-line control systems and, when they arrive with a variable timing, it is necessary to provide devices for storing said data using e.g. a buffer store and then the stored data is transferred to the printing mechanism of the printer.
The size of the storage devices or the buffer stores is determined on the one hand as a function of the maximum amount of data or dots which can be received per unit of time and on the other hand as a function of the printing speed of the printer. Thus, the size of the stores used increases as the printing speed decreases relative to the speed of the data received.
For limiting the size of the storage devices, without incurring the risk of any overflow thereof, numerous solutions have been proposed, which mostly reduce the printing time of the printers by increasing their printing speed.